Development of second-level connection method for large-size CPU package

This paper reports on second-level interconnection development for a large-scale Ball Grid Array (BGA) package. Generally, control of warpage becomes a problem as BGA packages become larger. To solve this problem, the following two measures were executed. The first was adoption of a low-temperature solder, and the second was warpage control using a heat spreader as a fixture. We were able to decrease the reflow temperature to 200°C by applying the low-temperature solder, and the effect was a warp reduction of 200 μm. Moreover, the shape of the heat spreader was optimized through a thermal-stress simulation, obtaining a warp reduction of 100 μm. Verification with a test vehicle was executed, no short/opening was observed, and the results of a thermal cycle test and simulation confirmed there was no problem in reliability.